1. Field of the Invention
The invention relates to turbofan gas turbine engine fans and in particular to fan sections having rows of staggered rugged (highly resistant to foreign object damage) blades and relatively less rugged lighter weight blades.
2. Description of Related Art
Large turbofan engines are particularly subject to foreign object damage, commonly referred to as FOD, caused by objects such as ice, hail, birds and runway debris. Fan blades are conventionally strengthened to avoid severe debilitating damage to the fan blades. This strengthening substantially increases the weight of the fan blades, disc, and supporting structure. The increased engine weight in turn increases the weight of the aircraft and requires more fuel to be burned.
Wide chord fan blades enhance the fan's performance and the engine's efficiency but pose additional problems in terms of blade weight while increasing the blade FOD resistance or ruggedness. Composite fan blades have been developed for use in slower speed high bypass ratio turbofan engines, having a bypass ratio of between 8 and 15, because of their weight advantage. However composite blades are relatively weak as compared to metal blades and have proven marginal in meeting impact requirements to demonstrate FOD resistance. Due to the strength and impact limitations of composites, composite fan bladed engine growth is limited. Composite fan blades are currently limited to engines having tip speeds below 1300 ft/sec and require leading edge guards on the fan blades for erosion protection. The present invention is particularly useful for wide chord turbofan fan blades.
Hollow titanium blades have been developed for use in turbofan engines of higher tip speeds (&gt;1300 ft/sec). Wide chord titanium blades are lightened by making them hollow but because of the aspects ratio and blade number, they are still not weight competitive with fan blades having a high aspect ratio and a conventional number of blades.
Hybrid blades (part composite and part titanium) have been proposed but they are still limited in terms of impact for the high speed application and are difficult and expensive to manufacture.